Creating your type's DNA
After you have completed good solid design and spacing of the 'o' and 'n', the next thing to do is to begin populating the font with letters whose structural characteristics provide useful DNA for making many of the other letters in the font.
It may be tempting to rush to populate your font as rapidly as possible with all the letters. You should resist this urge! The reason is that while 'n' and 'o' provide an excellent beginning to the foundation of the design, we need to establish the rest of it. Rapid expansion before this is done will mean that the whole project is harder to manage -- and takes longer than it needs to.
What else do we need for the foundation of our design? First, let us look at what we have with our 'n' and 'o.'
Although the 'o' is especially useful for working out the basic spacing, it is not going to help us design other characters; not necessarily even the 'b' or 'd'.
The letter 'n', on the other hand, is very useful because it helps making the m, h, and u. The other factor that we need to concern ourselves with in choosing letters for our foundation is how frequently the letter is used. A letter that is used a lot will help us make test words. Some of the letters may be chosen almost exclusively for this second reason.
The letters you choose don't have to be the same ones we suggest. They should simply have the characteristics being discussed. So, for instance, you may want to use "a d h e s i o n" to start with. This set of letters is used in the type design MA course at the University of Reading. But an alternative you may want to use is "v i d e o s p a n." The foundry Type Together uses this set themselves ,and when they teach type design. Either set has enough DNA to be meaningful, and it is relatively small so it is very manageable.
While it may be easiest to simply use one of the above sets of letters, you can also build your own.
If you do the latter, what set of letters should you pick to add to "n" and "o"? Consider the following.
a - The letter 'a' is also a very common. The 'a' may also be useful in anticipating what the terminals of s may be like.
d - The shape of 'd' can let you know quite a lot about the design of b, p and q.
e - In English and many other languages, the letter 'e' is especially common -- which makes it especially valuable. The shape of 'e' can also be used to begin the design of 'c.'
h - While 'h' can be built fairly rapidly from the 'n,' it also provides variety to the texture you want to test by offering an ascender.
i - Like 'e' the letter 'i' is also fairly common, and it has the benefit of letting you know a little bit about what 'j' is like. The shape of 'i' is also partly inferable from the shape of 'n.'
s - The letter 's' is a good one to add early on because it adds visual variety to the texture of letters you will be testing. The letter 's' is also unusually hard to get right, so starting on it early makes it more likely that you will be able to spend enough time to get it right by the end of the project. The terminals of 's' may sometimes be useful for anticipating what the terminals of a, c, f, j and y could be like.
v - The letter 'v' is useful for anticipating what the 'y' and 'w' may be like.
One you have these letters, it will be useful to spend time refining them by testing words that are made from them. As before with the 'n' and 'o' a great deal of attention should be paid to the spacing of the letters and the relationships of the counters to these spaces.
Build a test text
There are many resources for rapidly building your test text;
There is a Libre software solution provided by Dave Crossland at http://libretext.org
"Adhesion Text" was the first resource of this kind. It was made by Miguel Sousa: http://www.adhesiontext.com/
The foundry "Just Another Type Foundry" also offers an excellent resource: http://justanotherfoundry.com/generator